Clarify addE()-step from()/to() semantics in reference docs

[spmallette]

The AddE Step section of the reference documentation had a few gaps that made the from()/to() behavior hard to learn from the examples alone:

• Callouts 5 and 6 carried identical prose ("directed (detached) vertex references") even though they show fundamentally different patterns. g.V(vMarko).addE('knows').to(vPeter) uses the incoming traverser as the implicit from-vertex, whereas g.addE('knows').from(vMarko).to(vPeter) spawns the edge from g — where there is no traverser to act as a default endpoint, so both endpoints must be stated explicitly. The two callouts are now distinct and explain that distinction.
• The from()/to() argument forms were demonstrated but never stated as a rule. Added a short introductory note documenting the two canonical forms (a step-label string and a child traversal), and clarifying that passing a Vertex object directly is convenience syntax exposed by the language variant (e.g. Gremlin-Java, which resolves the Vertex to its id) rather than canonical Gremlin.
• The cumulative nature of the example block (each addE() persists and edge ids accumulate line to line) was never mentioned; added a note.
• Callout 2's stated edge direction was reversed: the query produces lop->josh and ripple->josh, but the prose read "from the josh-vertex to the lop- and ripple-vertices." Corrected.

Only the AddE Step section prose/callouts were touched; the live examples are unchanged and still render the same output.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.48%. Comparing base (a28cd1f) to head (c89e0a2).
⚠️ Report is 428 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3609      +/-   ##
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+ Coverage     76.35%   76.48%   +0.12%     
- Complexity    13424    14299     +875     
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  Files          1012     1036      +24     
  Lines         60341    64620    +4279     
  Branches       7075     7664     +589     
============================================
+ Hits          46076    49426    +3350     
- Misses        11548    12102     +554     
- Partials       2717     3092     +375     

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